Thermal process for improving the mechanical forming of magnesium alloys

ABSTRACT

A PROCESS FOR PRODUCING A MAGNESOUM ALLOY DIE EXTRUSION CAPABLE OF BEING SUBSEQUENTLY WORKED WITHOUT INCURRING INCIPIENT SURFACE CRACKING. THE PROCESS COMPRISING DIE EXTRUDING A MAGNESIUM ALLOY, QUENCHING THE EXTRUSION AS IT EXITS FROM THE DIE, AND ANNEALING THE EXTRUSION TO PROVIDE RANDOMLY ORIENTED GRAIN STRUCTURE AT LEAST AT THE SURFACE, THEREBY IMPROVING MECHANICAL WORKABILITY.

1973 R. o. KOEPLINGER EI'AL 3,709,745

THERMAL PRQCESS FOR IMPRQVING THE MECHANICAL FORMING 0F MAGNESIUM ALLOYS Filed Oct. 19, 1970 INVENTORQ Ronabfl. Kay/Myer John Pas 0k George 62 Foers/er :4 TTORNEYd" United States Patent O 3,709,745 THERMAL PROCESS FOR IMPROVING THE MECHANICAL FORMING OF MAGNESIUM ALLOYS Ronald D. Koeplinger, Saginaw, John F. Pashak, Linwood, and George S. Foerster, Midland, Mich., assignors to The Dow Chemical Company, Midland,

Mich.

Filed Oct. 19, 1970, Ser. No. 81,664 Int. Cl. C221? 1/06 US. Cl. 14811.5 M 12 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to metal working, but more specifically to the die extrusion of magnesium and alloys thereof. Even more particularly the invention pertains to the thermal preparation of die extruded magnesium alloys preceding working by the impact extrusion process.

Methods capable of making a fine grained extruded article of magnesium base alloys are described in US. Pat. 3,181,337, issued May 4, 1965. Therein it was necessary to spray coolant onto the extrusion through spray ports after said extrusion had passed through the die. A fine grained extruded form was thereby produced, but upon subsequent metal working in processes, such as impact extrusion, surface cracking occurred.

In US. Pat. 3,364,707 issued Jan. 23, 1968, a method of quenching an extrusion was described which utilized a forming member assembly comprising a first die section, at least one intermediate section, and an exit section. This method also provides a die extruded alloy article which is suitable for many purposes, but surface cracking did occur upon further working.

It is a primary object of this invention to provide a process which will improve the mechanical workability of extruded magnesium alloy products.

Other objects and advantages will become apparent from the discussion of the invention which follows.

The aforementioned objects and advantages have been realized in the current invention, which involves as a principal embodiment a process comprising extruding an alloy of magnesium into a shaped form, quenching said form substantially simultaneously with exit of said form from an extrusion die, and annealing said form, thereby producing an extrusion with improved mechanical workability. Quenching is considered to be substantially simultaneous with exit of the extrusion from the die when the quenched and annealed form can be impact extruded without cracking.

Even more particularly this invention comprises (a) extruding an alloy of magnesium through a shaped die cavity into a quench chamber assembly longitudinally adjacent and detachably secured to said die; (b) quenching the extrusion with a fluid emanating from a plurality of passages in the quench chamber, thereby cooling said extrusion and producing a fine grained structure therein; and (c) annealing said extrusion to form therein primarily randomly orientated grains at least at the surface. The extrusion so treated can be transversely sectioned into slugs and impact extruded without detrimental cracking.

7 3,709,745 Patented Jan. 9, 1973 ice Quenching of the extrusion may be accomplished utilizing quench apparatus of a number of configurations. Examples of such quenching means are depicted in the aforementioned U.S. Pats. 3,181,337 and 3,364,707.

The accompanying figure is a partial cross-section of still another quench chamber which can be used in the present method. A copending application bearing Ser. No. 81,663 filed Oct. 19, 1970 by George S. Foerster and Russell =E. Matthews entitled Fluid Quench Apparatus more fully describes this quench chamber.

Referring to this figure, the quench chamber assembly is longitudinally adjacent and preferably detachably secured at plane 1 to extrusion die 2, which contains an aperture 3, by an appropriate attaching means. Examples of a few suitable attaching means are bolting 16, extrusion ram pressure exerted against the die-quench chamber combination, and clamping. Quench chamber 4 is longitudinally enclosed by sleeve 5 and transversely by die 2 and aperture 3 on the inlet end and open-ended on the exit end. Sleeve 5 is interiorly of uniform dimension and exteriorly consists of a flange 6 adjacent to plane 1, a section of reduced wall thickness 7, and a section of greater outside diameter than section 7 to mate with a tension exerting restraining means capable of holding sleeve 5 in the proper position. For example a partially exteriorly threaded longitudinal section 8 in combination with an interiorly threaded collar 13 is a suitable restraining means. A plurality of spaced apart holes 9 from which a quench fluid emanates communicate radially and, optionally, diagonally through sleeve 5 to a longitudinally extending fluid distributing channel 10, which is peripherally disposed around and spaced apart from chamber 4 by sleeve 5. Channel 10 is bounded transversely by flange 6 and the partially exteriorly threaded section 8 and longitudinally by sleeve 5 and backer block 11. Backer block 11 is interiorly longitudinally uniform in dimension where it bounds fluid distributing chamber 10. A basically regularly shaped recess 12 extends transversely outwardly from the interior of said block 11 to accommodate flange 6. Backer block 11 is machined in a configuration to aflFord detachably mounting of said block to mate with die 2 along plane 1. The exterior periphery of block 11 is so shaped to afford placement of block 11 within the particular equipment to which the quench chamber assembly is aflixed. Sleeve 5 is detachably mounted to backer block 11 by inserting flange 6 in recess 12 and tightening an interiorly threaded collar 13 on the exteriorly threaded section 8 of sleeve 5. Inlet and optional outlet fluid passages 14 extend inwardly from the outlet end of block 11 and continuously extend to fluid distributing chamber 10. Said fluid passages 14 may be interiorly threaded toward the outlet end of the quench assembly to accept fluid source and return transmitting members.

Rapidly cooling or quenching the extruded article substantially coincident with the exit of the extrusion form the outlet end of the die provides a suitable structure to form the desired randomly orientated grain structure during annealing. The as-quenched internal structure is conducive to metal surface crack formation upon impact extrusion, when the die extruded slug is aligned in the impact die or mold so that working will be opposite the direction of the die extruding. It is theorized that this cracking is the result of a band of preferentially orientated grains present at the die extruded metal surface. Surface cracking occurs when the extruded form is impact extruded in a direction causing metal flow coincident with the slip planes of weakness formed by the preferentially orientated grains. However, in only this condition will subsequent annealing result in a desired micro-grain structure exhibiting a random grain structure at the metal surface and optionally throughout the cross-section of the extruded form. It is also hypothesized thatin addition to a randomly orientated grain structure, the twinning of the surface grains achievable by this process even further enhances the mechanical workability of the annealed meta1. Little reversion to the preferentially orientated grain structure will occur during annealing and, therefore, the criticality of locating the slug in the proper extrusion work direction is negated by this method.

PREFERRED EMBODIMENT In the process of the instant invention it is preferred that the alloy of magnesium to be extruded contain at least 50% magnesium. Examples such as ternary alloys of magnesium comprised of magnesium-thorium-rnanganese, such as, HM21 alloy, which is comprised of, by weight, about 1.5% to 2.5% thorium and about 0.45% to 1.1% manganese; magnesium-zinc-zirconium, such as ZK60A alloy which is comprised of, by weight, approximately 4.8% to 6.2% zinc and about 0.45% minimum zirconium; and magnesium-aluminum-zinc, such as, AZ31B alloy which is comprised of, by weight, about 2.5% to 3.5% aluminum, about 0.6% to 1.4% zinc, and about 0.2% minimum manganese will be benefited by the herein described process. The magnesium alloy most receptive to the benefits of this invention is AZ21 which is by weight comprised of about 1.6% to about 2.0% aluminum, about 1.0% to about 1.4% zinc, about 0.02% to about 0.15

' manganese, and about 0.15% to about 0.25% calcium.

By proper location of the fiuid spray holes in the quench chamber in close proximity to the die egress and optionally throughout the quench chamber the shaped extrusion may be quenched within about one second for about 1%" diameter rod and provide a suitable structure for annealing into random orientation. The time elapsed before quenching may vary somewhat depending upon the diameter or size of the shape die extruded. Exact time delays optimum for specific sizes can really be ascertained by examining a quenched specimen metallographically and determining the extent of grain growth occurring after extrusion. While size and number of the spaced apart holes or orifices are not critical, there must be adequate fiuid volume and velocity utilized to remove sufiicient heat to inhibit recrystallization of the extruded surface grains prior to the following annealing operation.

Economically and practically a quench fluid of Water has been found to be adequate. Other liquids or gases having a capacity for cooling rapidly magnesium-alloys from the extrusion temperatures can be used in lieu of Water.

Subsequent to quenching, the extruded form is annealed at a temperature within the range of about 600 F. to about 900 F. The temperature and holding time at said temperature must be suflicient to permit grain growth to achieve a substantially random grain orientation at least at the surface. Normally and preferably this holding period will be from about A to about 2 hours. Since room temperature aging is minimal in magnesium alloys the time interval between quenching and annealing is not critical.

The feasibility of this process is demonstrated by the following examples.

Example 1 Magnesium alloy AZ21 billet was preheated to a uniform temperature of 850 F. prior to extrusion into 0.935 inch diameter rod. Extrusion into this size was accomplished at a speed of seven feet per minute. The rod was quenched with a water spray of 6.5 pounds per minute Within 0.84 second or 1.2 inches of exit from the die orifice. Temperature of the metal was reduced by the quench water to 640 F. After annealing the 0.935 inch diameter rod for one hour at 850 F. it was transversely sectioned and impact extruded into battery cases without occurrance of obvious cracking.

Example 2 The practice of extrusion and annealing AZ21 alloy into 0.935 inch diameter rod was practiced as in Example 1. A quench Water flow rate of 13.5 pounds per minute was utilized in the example to achieve a rod temperature of F. upon exit from the quench apparatus. A satisfactory battery case was produced by impact extrusion of slugs from this rod.

Examples 3-5 Extrusion of AZ21 magnesium alloy was conducted by preheating the billet to temperatures varying from 600 F. to 850 F. and extruding at speeds from three to seven feet per minute. These variables did not alter the successful results upon impact extrusion. Rod 0.890 inch diameter was quenched 0.74 second from the die orifice, annealed, sectioned and impact extruded without occurrance of incipient surface cracking. An extrusion 1.245 inch diameter was water quenched 0.73 second from the die orifice, annealed, sectioned, and successfully impact extruded into battery cases.

In contrast to the above examples, similar 1.245 inch diameter AZ21 alloy extrusion was quenched in water 1.35 seconds from the die orifice, annealed, sectioned, an impact extruded as previously described. In this instance, though, the water quench was delayed excessively for the extrusion conditions and the impact extrusion, therefore, showed evidence of surface cracking.

As indicated by the aforementioned examples, when die extruded magnesium rod is spray quenched with sufficient coolant to reduce the surface temperature of said extrusion to a maximum of 640 F. within about 1 second, the quenching is substantially simultaneous with exit of the extrusion from the die and the extrusion can be impact extruded after annealing without cracking.

We claim:

1. A process comprising (a) die extruding an alloy of magnesium into a shaped form, (b) quenching said form substantially simultaneously with exit of said form from the extrusion die, and (c) annealing said form thereby producing an extrusion with primarily randomly orientated grains at least at the surface and improved mechanical workability.

2. The process of claim 1 wherein quenching is accomplished by water.

3. The process of claim 1 wherein the alloy extruded is comprised of, by weight, from about 1.6% to about 2.0% aluminum, about 1.0% to about 1.4% zinc, about 0.02% to about 0.15% manganese, and about 0.15% to about 0.25% calcium.

4. The process of claim 3 wherein said annealing is accomplished by maintaining said extrusion at about 600 F. to about 900 F. for about A to about 2 hours.

5. The process of claim 1 including the steps of (d) transversely sectioning the annealed extrusion into slugs and (e) impact extruding said slugs.

6. The process which comprises (a) extruding an alloy of magnesium through a shaped die cavity into a quench chamber assembly longitudinally adjacent and detachably secured to said die, (b) quenching an extrusion with a fluid emanating from a plurality of passages in said quench chamber, thereby cooling said extrusion and providing a fine grained structure therein, and (c) annealing said extrusion to form therein primarily randomly orientated grains at least at the surface.

7. The process of claim 6 wherein the alloy extruded is comprised of, by weight, from about 1.6% to about 2.0% aluminum, about 1.0% to about 1.4% zinc, about 0.02% to about 0.15 manganese, and about 0.15 to about 0.25 calcium.

8. The process of claim 6 wherein quenching is accomplished substantially simultaneously with exit of said extrusion from the die.

9. The process of claim 6 wherein quenching is accomplished by water.

10. The process of claim 6 including the steps of (d) transversely sectioning the annealed extrusion into slugs and (e) impact extruding said slugs.

11. The process which comprises (a) extruding an alloy of magnesium through a shaped die cavity into a quench chamber assembly longitudinally adjacent and detachably secured to said die, (b) quenching an extrusion with a fluid emanating from a plurality of holes communicating with a quench chamber through a quench assembly sleeve and into a longitudinally extending fluid distributing channel peripherally disposed around and spaced apart from said chamber by said sleeve and further longitudinally bounded by a backer block, thereby cooling said extrusion and providing a small grained structure therein, and (c) annealing said extrusion to' form therein primarily randomly orientated grains at least at the surface, (d)

transversely sectioning said extrusion, and (e) impact extruding a section of the extrusion.

12. The process of claim 11 wherein quenching is accomplished by water.

References Cited UNITED STATES PATENTS 3,333,956 8/1967 Foerster 148-'-12.7 X 3,245,843 4/1966 Brackett, Jr 148-1l.5 3,131,095 4/1964 Hershey et al. 1481l.5 X 3,320,055 5/1967 Foerster 148-l1.5 X 1,923,592 8/1933 Schmidt et a1. 148l1.5 M

5 FOREIGN PATENTS 1,187,305 4/1970 Great Britain 148-12.7

WAYLAND W. STALLARD, Primary Examiner 

